What? A pair is two. Maybe this is supposed to be pear? Or maybe pare?
Think about this.
Get your head out of the gutter. This is not about an Ménage à trois.
But we have all done it.
You meet a person. Boy or girl. You become friends. You hang around together and become a pair.
Then one of you meets another person.
Perhaps introduce that person to the first person.
So why is this the perfect pair?
With two people, there are mutual agreements and easily shared conversations, but a third person brings tension.
A pair can become romantically involved or become couples with another pair, but not with a trio.
The third person is the odd person out raising conflict to any situation.
Three is the Perfect Pair.